Process for plasmonic-based high resolution color printing

ABSTRACT

A process for plasmonic-based high resolution color printing is provided. The process includes a) providing a nanostructured substrate surface having a reverse structure geometry comprised of nanopits and nanoposts on a support, and b) forming a conformal continuous metal coating over the nanostructured substrate surface to generate a continuous metal film, the continuous metal film defining nanostructures for the plasmonic-based high resolution color printing, wherein a periodicity of the nanostructures is equal to or less than a diffraction limit of visible light. A nanostructured metal film or metal-film coated support obtained by the process and a method for generating a color image are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a U.S. National Phase Application under 35U.S.C. § 371 of International Application No. PCT/SG2015/050071, filedon 14 Apr. 2015, entitled PROCESS FOR PLASMONIC-BASED HIGH RESOLUTIONCOLOR PRINTING, which claims the benefit of priority of Singapore PatentApplication No. 10201401610R, filed on 17 Apr. 2014, the content ofwhich was incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

Various embodiments relate to a process for plasmonic-based highresolution color printing, a nanostructured metal film or metal-filmcoated support obtained by the process, and a method for generating acolor image.

BACKGROUND

The landscape of color printing technologies is currently dominated bycolorant pigmentation methods involving production of a range ofdifferent colors by overlaying primary color dyes in differentproportions onto a substrate to create a full color print. While suchpigment-based approaches constitute the current industry standard,resolution of the printed image is constrained by spot size of the dyedroplets—typically in the micro-sized range. Further limitations includecolor bleeding between adjacent droplets, and fading of color pigmentsresulting from breakdown of molecules when exposed to UV radiation orintense heat. These fundamentally limit the resolution and durability ofpigment-based printing technologies.

Approaches for fabricating high-resolution color prints where colorcreation is achieved through excitation of plasmonic resonances in metalnanostructures have been developed. They present several uniqueadvantages over conventional pigment-based printing technologies, suchas (1) vibrant color prints at the ultimate resolution of the opticaldiffraction limit; (2) multicolor palette from a single metalevaporation step on a nanostructured substrate; and (3) broad spectrumof colors determined by nanostructure design, thereby eliminatingundesirable effects of color bleeding associated with pigment-basedprinting technologies.

Notwithstanding the above, most of the plasmonic-based printing methodsare based on transmitted light, thereby limiting their application totransparent substrates. They may also require special illuminationtechniques, and/or specific viewing angles to view the prints.

One of the plasmonic-based printing methods developed involvesdepositing layers of metal on arrays of hydrogen silsesquioxanenanoposts to form isolated plasmonic nanodisks. Formation of theisolated plasmonic nanodisks requires height of the nanoposts to be muchlarger than thickness of the metal layers. The nanodisks are elevatedabove equally sized holes formed on a backreflector which functions as amirror to increase scattering intensity of the nanodisks. Interaction ofthe nanodisks with the backreflector thus results in extinction ofspecific wavelengths in the visible range, seen as colors reflected offthe nanodisks which may be observed in an optical microscope. Theisolated plasmonic nanodisks exhibit distinct resonances which induceextinction of specific wavelengths in the visible spectra, and manifestas colors reflected off the nanodisks that may be observed in an opticalmicroscope.

This method, however, involves more steps in the fabrication processgiven the composite structure which typically includes a metal and adielectric. Use of thin metal layers of about 20 nm in these plasmonicpixels also implies the need for fine control of the metal thicknessduring the metallisation process. In particular, the method may requireuse of expensive equipment and special processing conditions, such aselectron beam evaporation in a vacuum chamber, so as to allowdirectional line-of-sight deposition for forming the isolated metalnanostructures, while leaving vertical sidewalls of the nanopostsuncoated.

The isolated metal nanostructures were deemed necessary for generationof plasmonic colors. Conventional metal deposition methods such asdirect metal deposition (DMD), spray deposition, and conformal metaldeposition, cannot be used since they create continuous metal films andare not able to generate the isolated metal nanostructures. Thispresents a steep barrier of entry for commercial use. There were also noarchitectures that may be used to generate plasmonic colors which arecompatible with the conventional metal deposition methods.

In view of the above, there exists a need for an improved method forplasmonic-based high resolution color printing that overcomes or atleast alleviates one or more of the above-mentioned problems.

SUMMARY

In a first aspect, a process for plasmonic-based high resolution colorprinting is provided. The process comprises

-   -   a) providing a nanostructured substrate surface having a reverse        structure geometry comprised of nanopits and nanoposts on a        support, and    -   b) forming a conformal continuous metal coating over the        nanostructured substrate surface to generate a continuous metal        film, the continuous metal film defining nanostructures for the        plasmonic-based high resolution color printing, wherein a        periodicity of the nanostructures is equal to or less than a        diffraction limit of visible light.

In a second aspect, a nanostructured metal film or metal-film coatedsupport obtained by a process according to the first aspect is provided.

In a third aspect, a method for generating a color image is provided.The method comprises irradiating a nanostructured metal film ormetal-film coated support obtained by a process according to the firstaspect with visible light.

In a fourth aspect, use of a method according to the third aspect insecurity tagging, anti-counterfeiting, display, and/or data-storage isprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the detaileddescription when considered in conjunction with the non-limitingexamples and the accompanying drawings, in which:

FIG. 1 depicts state of the art nanostructures for plasmonic colorprinting, where (a) is a schematic diagram of nanopost arrays forming‘plasmonic pixels’; (b) is an optical microscope image of a section of acolor print formed using ‘plasmonic pixels’. Insert of (b) is shown in(c), which is a scanning electron microscopy (SEM) image of sectionindicated within the dotted black box. Scale bars: (b) and (c) 500 nm.

FIG. 2 shows (A) a cross-sectional view of a nanostructured substratesurface 220 having a reverse structure geometry comprised of nanopits223 and nanoposts 221 on a support 225. s1 denotes distance betweenadjacent nanoposts 221; d1 denotes a cross sectional dimension ofnanopost 221; and h1 denotes height of nanopost 221. Height of nanopost221, h1, corresponds to depth of adjacent nanopit 223. In (B), besidesnanostructured substrate surface 220, a cross-sectional view of acontinuous metal film 240 defining nanostructures for theplasmonic-based high resolution color printing is additionally shown. Inthe embodiment shown, a conformal continuous metal coating 245 is formedover the nanostructured substrate surface 220, with subsequentseparation from the nanostructured substrate surface 220. The conformalcontinuous metal coating 245 fills the nanopits 223 of thenanostructured substrate surface 220 to result in formation of nanoposts241 on the continuous metal film 240, while nanoposts 221 of thenanostructured substrate surface 220 translate into nanopits 243 on thecontinuous metal film 240. d2 denotes a cross sectional dimension ofnanopost 241 on the continuous metal film 240, and may have a lengthcorresponding to s1; s2 denotes distance between adjacent nanoposts 241on the continuous metal film 240, and may have a length corresponding tod1; h2 denotes height of nanopost 241 on the continuous metal film 240,and corresponds to depth of adjacent nanopit 243. h2 may have a lengthcorresponding to h1.

FIG. 3 shows designs for high-resolution plasmonic color printing onsolid metal films and surfaces, where (A) shows metal nanostructuresformed on an optically opaque metal film and coated with a polymer film;and (B) shows bulk metal substrate patterned with nanostructures. In(A), a pre-patterned polymer film may be fabricated using patterningtechniques such as nanoimprint lithography as shown in (i). Next, acontinuous metal film may be formed on the patterned polymer surface byprint deposition as shown in (ii). This may be followed by an annealingstep, to yield the nanostructure illustrated in FIG. 3(A)(iii)—anoptically opaque metal film patterned with either nanopits or nanopostsand coated with a polymer film. Light may be irradiated on thenanostructured metal film through the polymer film to generate a colorimage. This particular approach may be envisioned for application inplastic notes currency. In (B), a hard master mould may first befabricated using suitable patterning methods such as electron beamlithography (EBL) as shown in (i). Subsequently, minting of a metalsurface or deposition of additional metals onto the metal surface bymeans of electroplating may be carried out to achieve a bulk state ofmetal as shown in (ii). Finally, the nanostructure comprising a bulkmetal substrate patterned with either nanopits or nanoposts may bede-molded or separated from the mold as shown in (iii). Light may beirradiated on the metal film to generate a color image as shown in (iv).This approach may find application in coin currency.

FIG. 4 shows preliminary investigations on nanodisk structures formed ona continuous aluminum layer, where (A) is a schematic diagram ofaluminum nanodisk structures formed on an optically thick aluminum layerand coated with a polymer; and (B) is a graph showing simulatedreflection spectra of aluminum nanodisk structures, for (i) d=140 nm;(ii) d=160 nm; (iii) d=180 nm; and (iv) d=200 nm.

FIG. 5 shows preliminary results on nanoscale groove gold surfaces forgap plasmon-based color printing, where (A) is schematic diagram ofnanoscale groove gold surfaces; (B) are optical microscope image offabricated square, hexagonal, and triangular grooves; (C) to (E) arescanning electron microscopy images of fabricated (C) square, (D)hexagonal, and (E) triangular grooves on a gold surface; (F) is a graphshowing measured relative reflectance spectrum for the respectivegrooves; and (G) is a graph showing simulated reflectance spectrum forrectangular grooves with gaps varying from 9 nm to 17 nm. Scale bars:(B) 200 μm; (C) to (E) 500 nm.

FIG. 6 depicts schematic diagrams showing (A) irradiation of light onmetal nanostructures formed on an optically opaque metal film and coatedwith a polymer film according to embodiments; and (B) irradiation oflight on metal nanostructures formed on an optically opaque metal filmaccording to embodiments.

DETAILED DESCRIPTION

In a first aspect, a process for plasmonic-based high resolution colorprinting is provided.

As used herein, the term “plasmonic” or “plasmonic-based” may mean orrefer to use of incident wave, such as light, of a particular frequencyor wavelength range to cause excitation of free electrons in the metalsurface being irradiated upon, which may cause a drop in reflectivity ofthe metal as energy of the incident wave is coupled into plasmon modesinstead of being reflected by the metal. A certain range of wavelengthsof light in the visible range may be absorbed by the metal surface toallow observation of color(s) reflected from the metal surface. Theplasmon modes may include surface plasmon modes which propagate alongthe surface of the metal or bulk plasmon modes which propagate withinthe metal.

Advantageously, the process disclosed herein provides for a broad rangeof colors using economical, scalable approaches for metal deposition.Expensive equipment or processing under vacuum conditions required instate of the art methods is not required. This provides a significantreduction in cost and time needed to fabricate color images byplasmonic-based printing, further in view that cheap and/or earthabundant materials such as aluminum may be used. Instead of coating asubstrate and nanoposts with metal using vacuum-based evaporativedeposition techniques, solid metal nanoposts may be mass produced and acontinuous metal film comprising nanopost structures may be generated,rendering easier adoption of processes in industry.

Plasmonic resonances may be achieved on continuous metal films toachieve a broad range of colors, even in extreme cases where the metalfilms are thick and opaque. In embodiments disclosed herein,nano-textured surfaces on a bulk piece of metal patterned withnanostructures comprising nanoposts and/or nanopits are able to exhibitcolor. A range of different colors in color images may be determinedsolely by patterning to form an array of nanostructures and with asingle color-producing material in the form of a continuous metal film.

The color image formed using methods disclosed herein is observablebased on the reflection of light from the nanostructures, in a similarway of viewing a photograph by way of reflection of light from thephotograph. Excitation of the plasmonic resonances from the nanometallicstructures may manifest as arrays of different colors which are visiblein bright-field microscopy.

The process includes providing a nanostructured substrate surface havinga reverse structure geometry comprised of nanopits and nanoposts on asupport, and forming a conformal continuous metal coating over thenanostructured substrate surface to generate a continuous metal film.The continuous metal film defines nanostructures for the plasmonic-basedhigh resolution color printing, wherein a periodicity of thenanostructures is equal to or less than a diffraction limit of visiblelight.

The term “nanostructure” as used herein may have a size in at least onedimension in the nanometer (nm) range, for example, a range between 1 nmand 500 nm, e.g. a range between 1 nm and 200 nm, a range between 1 nmand 100 nm, a range between 10 nm and 100 nm, or a range between 50 nmand 100 nm.

In various embodiments, the nanostructures are in the form of nanopitsand nanoposts on a support.

The term “nanopost” may include a reference to a nanocolumn, a nanotube,a nanopillar or the like, and may refer to an elongate structure thatextends from a surface of the support and having a size, such as heightand/or diameter, in the nanometer (nm) range. Each nanopost may be ananostructure with a columniform shape.

The term “nanopit” may include a reference to a nanohole or the like,and refers to a pit having a size, such as depth and/or diameter, in atleast one dimension in the nanometer (nm) range.

The nanostructured substrate surface has a reverse structure geometrycomprised of nanopits and nanoposts on a support. As used herein, theterm “reverse structure geometry” refers to the nanostructured substratesurface having features which are in reverse, or opposed, to thefeatures to be formed on the continuous metal film. In this regard, thenanostructured substrate surface may function like a mold. By forming aconformal continuous metal coating over the nanostructured substratesurface to generate a continuous metal film, the metal coating may fillin the nanopits on the support. In so doing, nanopits on the supporttranslates into nanoposts on the continuous metal film. Nanoposts on thesupport, on the other hand, translate into nanopits on the continuousmetal film.

The support may comprise or consist of a material selected from thegroup consisting of a polymer, a metal, a metalloid, and combinationsthereof. In some embodiments, the support comprises or consists of apolymer. For example, the polymer may be polycarbonate, biaxiallyoriented polypropylene, combinations thereof, or copolymers thereof.

Providing the nanostructured substrate surface may comprise patterningthe support by any suitable method, such as a method selected from thegroup consisting of nanoimprint lithography, electron beam lithography,photolithography, and combinations thereof. Other methods that are ableto form the nanostructured substrate surface, though not explicitlystated herein, may also be used. In specific embodiments, providing thenanostructured substrate surface comprises patterning the support bynanoimprint lithography.

The process includes forming a conformal continuous metal coating overthe nanostructured substrate surface to generate a continuous metalfilm. Generally, any method that is able to form the conformalcontinuous metal coating, which may be carried out under non-vacuumconditions such as ambient conditions, may be used. In some embodiments,forming the conformal continuous metal coating is carried out by amethod selected from the group consisting of vapor deposition,sputtering, casting, coating, electroplating, and combinations thereof.

The continuous metal film may comprise or consist of a metal such asgold, silver, copper, and/or aluminum.

In some embodiments, the continuous metal film comprises or consists ofaluminum. Advantageously, aluminum is earth abundant hence presents acheaper option as compared to plasmonic color printing materials such asgold and silver.

The process may further include separating the continuous metal filmfrom the nanostructured substrate surface. This may be carried out, forexample, by lifting off or peeling the continuous metal film from thenanostructured substrate. In embodiments where the support is at leastsubstantially transparent to visible light or is optically transmissive,such as in the case of polycarbonate, the support may not need to beseparated from the continuous metal film, since it may not hinderirradiation of light onto the continuous metal film and/or observationof the color(s) reflected from the plasmonic nanostructures on thecontinuous metal film.

The continuous metal film defines nanostructures for the plasmonic-basedhigh resolution color printing. As mentioned above, the nanostructuredsubstrate surface may function like a mold, so that patterns present onthe nanostructured substrate surface may be transferred to thecontinuous metal film. For example, in conformally depositing acontinuous metal coating over the nanostructured substrate surface togenerate a continuous metal film, the metal coating may fill in thenanopits on the support. In so doing, nanopits on the support translatesinto nanoposts on the continuous metal film. Nanoposts on the support,on the other hand, translate into nanopits on the continuous metal film.The continuous metal film may thus comprise nanostructures havingtopography or patterns which are in reverse to the features on thenanostructured substrate surface. Dimensions of nanoposts and nanopitson the nanostructured substrate surface may be tailored or adapted toobtain nanostructures of specific dimensions on the continuous metalfilm for plasmonic-based high resolution color printing.

In various embodiments, the nanostructures on the continuous metal filmcomprise nanopits and nanorods, which are in reverse structure geometryto the nanostructured substrate surface.

The nanostructures on the continuous metal film may be arranged at leastsubstantially vertically or perpendicularly to a surface of thecontinuous metal film. However, it should be appreciated that any one ormore or all of the spaced apart nanoposts may be arranged slightlyangled to the surface, for example about 1° to 10° from an axis definedperpendicularly to the surface.

The nanostructures on the continuous metal film may have an aspect ratiogreater than 0.25. The term “aspect ratio” as applied to a nanostructuremay mean a ratio of the length (or height or depth) to the width (orcross sectional dimension) of the nanostructure. The length-to-widthaspect ratio of the nanostructure represents the proportionalrelationship between its length and its width.

For example, the nanostructures on the continuous metal film may have anaspect ratio between 0.25 and 2, between 0.25 and 1.5, between 0.25 and1, between 0.25 and 0.5, between 0.5 and 2, between 1 and 2, or between1.5 and 2, for example an aspect ratio of 0.25 where the length of thenanostructure is a quarter of the dimension of its width, an aspectratio of 1 where the length of the nanostructure is the same as itswidth, or an aspect ratio of 2 where the length of the nanostructure istwice as much compared to its width.

In various embodiments, each nanostructure may have an aspect ratiogreater than 1, with a length/height/depth that is larger than a width(e.g. a cross sectional dimension) of the nanostructure, i.e. thenanostructure has a dimension in the longitudinal axis of thenanostructure that is larger than a dimension in the transverse axis(perpendicular to the longitudinal axis) of the nanostructure.Therefore, the nanostructure may be very long in length or height whilebeing very short in diameter or width.

Height of each nanostructure may refer to a maximal distance of thenanostructure as measured from a base of the nanostructure to its tip.For example, height of nanoposts on the continuous metal film may bemeasured from base of the nanopost to its tip. In various embodiments,height of a nanopost on the continuous metal film corresponds to depthof an adjacent nanopit on the continuous metal film.

Height of each nanostructure may be 200 nm or less, such as in the rangeof about 10 nm to about 200 nm, about 50 nm to about 200 nm, about 100nm to about 200 nm, about 150 nm to about 200 nm, about 50 nm to about150 nm, about 100 nm to about 150 nm, about 80 nm to about 120 nm, orabout 90 nm to about 170 nm. In some embodiments, height of eachnanostructure is in the range of about 100 nm to about 200 nm.

Each nanostructure may have a cross sectional dimension in the range ofabout 10 nm to about 250 nm. The term “cross sectional dimension” maymean a dimension of a cross section of the nanostructure defined along atransverse axis (perpendicular to the longitudinal axis) of thenanostructure.

For example, each nanostructure may have a cross sectional dimension inthe range of about 10 nm to about 200 nm, about 10 nm to about 100 nm,about 10 nm to about 50 nm, about 100 nm to about 250 nm, or about 50 nmto about 200 nm.

The nanostructures may have different cross sectional dimensions. Inother words, each or some of the nanostructures may have a differentcross sectional dimension compared to other nanostructures. For example,some nanostructures may have a cross sectional dimension, d1, while someother nanostructures may have a different cross sectional dimension, d2.Furthermore, a cluster of nanostructures at one area/region of thesupport may have a cross sectional dimension that is different fromanother cluster of nanostructures at another area/region of the support.The cross sectional dimensions of the nanostructures may depend on thecolor(s) that is to be produced.

Each nanostructure may have a cross sectional shape that is a square, arectangle, a circle, an ellipse, a triangle, a hexagon, or an octagon, apolygon, to name only a few. The term “cross sectional shape” may mean ashape of a cross section of the nanostructure defined along a transverseaxis (perpendicular to the longitudinal axis) of the nanostructure.

In various embodiments, the nanostructures may have different crosssectional shapes. In other words, each or some of the nanostructures mayhave a different cross sectional shape compared to other nanostructures.For example, some nanostructures may have a circular cross sectionalshape, while some other nanostructures may have a triangular crosssectional shape. Furthermore, a cluster of nanostructures at onearea/region of the support may have a cross sectional shape that isdifferent from another cluster of nanostructures at another area/regionof the support. The cross sectional shapes of the plurality ofnanostructures may depend on the color(s) that is to be produced.

Two adjacent (or neighbor) nanostructures may be spaced apart by adistance or spacing or gap, s, of between about 20 nm and about 300 nm,e.g. between about 20 nm and about 200 nm, between about 20 nm and about100 nm, between about 20 nm and about 50 nm, between about 50 nm andabout 300 nm, or between about 100 nm and about 200 nm, for example aspacing of about 20 nm, about 50 nm, about 100 nm or about 200 nm.

In various embodiments, adjacent nanostructures may be spaced apart bydifferent distances, s, thereby changing the areal densities of thenanostructures on the continuous metal film. In the context of variousembodiments, the term “areal density” may refer to the density orpopulation or number of nanostructures at a particular area of thesubstrate. In other words, some adjacent nanostructures may have adifferent spacing, s, and therefore also the pitch, p, compared to otheradjacent nanostructures.

For example, some adjacent nanostructures may have a spacing, s1, orpitch, p1, while some other adjacent nanostructures may have a differentspacing, s2, or pitch, p2. Furthermore, a cluster of adjacentnanostructures at one area/region of the continuous metal film may havea spacing or a pitch, and therefore also an areal density ofnanostructures, that is different from another cluster of adjacentnanostructures at another area/region of the continuous metal film. Thespacings or pitches of the plurality of nanostructures, and thereforealso the areal densities of the nanostructures on the continuous metalfilm, may depend on the color(s) and/or the intensity of the color(s)that is to be produced.

As used herein, the terms “pitch” and “periodicity” are usedinterchangeably, and refer to the distance between the respectivecentres of adjacent nanostructures. The pitch, p, may be defined as(d+s), where d is the cross sectional dimension of a nanostructures, ands is the distance or spacing between two adjacent (or neighbor)nanostructures.

The periodicity of the nanostructures on the continuous metal film isequal to or less than a diffraction limit of visible light. As usedherein, the term “diffraction limit” refers to Abbe's classicaldiffraction limit, which states that the minimum resolvable distancebetween two closely spaced objects is at best half the wavelength usedfor imaging. Hence, in various embodiments where wavelength of 1000 nmis used, a diffraction limit of visible light as used herein may referto a distance or pitch of 500 nm between two objects. In someembodiments, 500 nm is taken as the mid-spectrum wavelength for visiblelight, therefore, a diffraction limit of visible light as used hereinmay refer to a distance or pitch of 250 nm between two objects.

In various embodiments, the periodicity is 500 nm or less. For example,the periodicity may be in the range of about 5 nm to about 500 nm, suchas about 50 nm to about 500 nm, about 100 nm to about 500 nm, about 200nm to about 500 nm, about 300 nm to about 500 nm, about 5 nm to about350 nm, about 5 nm to about 250 nm, about 5 nm to about 150 nm, about 50nm to about 300 nm, or about 100 nm to about 200 nm.

In some embodiments, the periodicity is 250 nm or less. For example, theperiodicity may be in the range of about 5 nm to about 250 nm, such asabout 5 nm to about 200 nm, about 5 nm to about 150 nm, about 5 nm toabout 100 nm, about 5 nm to about 50 nm, about 25 nm to about 250 nm,about 50 nm to about 250 nm, about 100 nm to about 250 nm, about 150 nmto about 250 nm, about 200 nm to about 250 nm, about 50 nm to about 200nm, about 100 nm to about 200 nm, about 80 nm to about 180 nm, or about120 nm to about 220 nm.

Adjacent nanostructures may be spaced apart by a distance, s, which isat least substantially the same. Accordingly, the pitch or periodicity,p, may be at least substantially the same.

Variation in colors may be obtained by tuning plasmonic resonances ofthe nanostructures by changing size and/or shape of the nanostructures.When the nanostructures are irradiated with light, these plasmonicresonances determine the color(s) of light that is absorbed by thenanostructures, thereby allowing a range of different colors to bereflected and hence observable.

In various embodiments, nanostructures such as nanoposts are used forcolor generation. Variety of colors that may be observed may becontrolled by varying the cross sectional dimension/size and the crosssectional shape of the nanoposts, as well as the pitch between adjacentnanoposts, for example, so as to vary the wavelength of light that isabsorbed by the nanostructures. Therefore, an exposure layout of apattern corresponding to the arrays of nanoposts may be designed inorder to produce a significantly miniaturized colored photo/image of anoriginal image.

The color image of various embodiments may be made up of a plurality ofpixels, where each pixel may be defined either by a single structure(e.g. including one or more filters) or a cluster of structures fordisplaying red, green and blue colors (RGB). Each structure may betermed as “plixel” (a combination of the words “plasmonic” and “pixel”),and made up of a plurality of plasmonic nanostructures. Using methodsdisclosed herein, high resolution color printing of up to about 10⁵ dpimay be achieved. This resolution may be varied by controlling size ofeach plixel. For example, the resolution may be further increased byreducing size of each plixel.

Within each pixel area, the plurality of nanostructures or nanoposts maybe positioned in a regular or uniform arrangement, or may be positionedrandomly but maintaining the spacing, s, between adjacent nanostructuresat approximately equal distance.

The color image may be observable using a bright-field illumination. Invarious embodiments, bright-field color prints or images withresolutions up to the optical diffraction limit may be obtained. Colorinformation may be encoded in the dimensional parameters and positionsof nanostructures, so that tuning of the plasmonic resonance of thenanostructures may determine the colors of the individual pixels. Inembodiments where the patterned area is sufficiently large, such asgreater than 100 μm×100 μm, the colors may be visible to the naked eye.

In the context of various embodiments, the cross sectional dimension,and/or the cross sectional shape of a nanostructure, and/or the spacing(or pitch) of adjacent nanostructures, and/or the material of thecontinuous metal film, and therefore also of the plasmonicnanostructures, may be changed depending on the color(s) to be produced.In other words, the color(s) that is produced or reflected by aplasmonic nanostructure may correspond to its cross sectional dimensionand/or its cross sectional shape and/or its distance from anotherplasmonic nanostructure and/or the material of the plasmonicnanostructure or a combination of any two, three or all of thesefeatures.

In addition to achieving high resolution, the use of plasmonicresonators or nanostructures may also provide secondary degrees offreedom to color creation, including polarization dependence. Furtherimprovements in resolution and color perception may be achieved by usingdifferent geometries and/or smaller numbers of plasmonic nanostructuresper pixel area.

Various embodiments refer in a second aspect to a nanostructured metalfilm or metal-film coated support obtained by a process according to thefirst aspect.

As mentioned above, the process according to the first aspect mayinclude separating the continuous metal film from the nanostructuredsubstrate surface, which may be carried out by lifting off or peelingthe continuous metal film from the nanostructured substrate, forexample. Hence, the continuous metal film may be separated from thenanostructured substrate surface to obtain the nanostructured metalfilm.

Otherwise, the continuous metal film may remain on the nanostructuredsubstrate surface to form a metal-film coated support. As mentionedabove, the continuous metal film may not need to be separated from thenanostructured substrate surface, if the support is at leastsubstantially transparent to visible light or is optically transmissive.

Various embodiments refer in a third aspect to a method for generating acolor image. The method comprises irradiating the nanostructured metalfilm or metal-film coated support obtained by a process according to thefirst aspect with visible light.

When the nanostructures are irradiated with light, plasmonic resonancesgenerated by the nanostructures on the nanostructured metal film ormetal-film coated support may determine the color(s) of light that isabsorbed by the nanostructures, thereby allowing a range of differentcolors to be reflected and hence observable.

Various embodiments refer in a fourth aspect to use of a methodaccording to the third aspect in security tagging, anti-counterfeiting,display, and/or data-storage.

The approach or color-mapping technique of the various embodiments maybe applied to create a full-color image or micro-image with high levelsof details, with both sharp color changes and fine tonal variations.

As the images are of high resolutions, the images cannot be printed withexisting color printers, which have a resolution limit of about 10,000dpi (dots per inch). In contrast, the color micro-photography techniqueof various embodiments may produce a pixel density as high as about100,000 dpi, for example where the individual color elements (or pixels)have a pitch of about 250 nm. Therefore, the technique of variousembodiments enable the printing of color micro-images ormicro-photographs having a resolution with more than an order ofmagnitude higher than state-of-the-art printers.

As a result of the high levels of details/resolutions due to thehigh-level of technology involved in creating the micro-image, such animage may be employed as a highly secure (e.g. anti-fraud) element onitems such as smart cards/credit cards, or as security features oncurrency notes or coins.

Given the wide range of color tunability, plasmonics color printingtechnology disclosed herein is ideal for high-tech security tagging andanticounterfeiting applications, for example, in the form of anti-fraudelements on currency and smart cards/credit cards. It also has theunique capacity to engineer high-resolution color pixels which encodemultiple layers of optical information within the same area of volume.Use of plasmonic resonances has added advantage that the plasmoniccolors do not degrade over time.

Besides the above-mentioned applications, the micro-images produced maybe used for steganography, nanoscale optical filters and high-densityspectrally encoded optical data storage.

Hereinafter, the present invention will be described more fully withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Inthe drawings, lengths and sizes of layers and regions may be exaggeratedfor clarity.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. The terminology used hereinis for the purpose of describing particular embodiments only and is notintended to be limiting of the invention. As used herein, the singularforms “a”, “an” and “the” are intended to include the plural forms aswell, unless the context clearly indicates otherwise. It will be furtherunderstood that the terms “comprises” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

The invention illustratively described herein may suitably be practicedin the absence of any element or elements, limitation or limitations,not specifically disclosed herein. Thus, for example, the terms“comprising”, “including”, “containing”, etc. shall be read expansivelyand without limitation. Additionally, the terms and expressions employedherein have been used as terms of description and not of limitation, andthere is no intention in the use of such terms and expressions ofexcluding any equivalents of the features shown and described orportions thereof, but it is recognized that various modifications arepossible within the scope of the invention claimed. Thus, it should beunderstood that although the present invention has been specificallydisclosed by preferred embodiments and optional features, modificationand variation of the inventions embodied therein herein disclosed may beresorted to by those skilled in the art, and that such modifications andvariations are considered to be within the scope of this invention.

The invention has been described broadly and generically herein. Each ofthe narrower species and subgeneric groupings falling within the genericdisclosure also form part of the invention. This includes the genericdescription of the invention with a proviso or negative limitationremoving any subject matter from the genus, regardless of whether or notthe excised material is specifically recited herein.

Other embodiments are within the following claims and non-limitingexamples. In addition, where features or aspects of the invention aredescribed in terms of Markush groups, those skilled in the art willrecognize that the invention is also thereby described in terms of anyindividual member or subgroup of members of the Markush group.

EXPERIMENTAL SECTION

An approach for fabricating plasmonic color pixel and design forplasmonic color pixels based on continuous instead of isolatednanostructures are disclosed herein. There may be two approaches towardsachieving this: (1) by depositing a conformal metal coating over apre-patterned surface, or (2) by patterning continuous metal structureson a solid, optically thick metal substrate, such as that shown in FIG.3.

This approach uses continuous nanostructures instead of isolatednanostructures as the basic element to form a printed color image. Thesenew types of pixels allows color imaging to be achieved either through athick conformal metal coating on a pre-patterned surface, or by directlypatterning on a solid, optically thick piece of metal. Using methodsdisclosed herein, color images at the optical diffraction limit may beprinted directly onto metal surfaces, such as coins and other metalfinishes, as well as provide a cost-effective and economically feasibleprocess for depositing metal onto plastic films, for example, forapplication towards printing of plastic notes currency.

Advantageously, only a single plasmonic metal is required, thus reducingthe need for extra fabrication steps. As compared to state of the artplasmonic-based printing methods, dielectric spacers for isolating theresonances generated in the isolated nanostructures are not required.This is surprising because initial experiments suggested that no colorwas generated when disks were placed directly on a reflectivenon-metallic surface (e.g. silicon) without using a dielectric, in anarray of short nanopillars and shallow nanopits on a metal surface. Assuch, it is counter-intuitive that a spectrum of different colors may beachieved using continuous nanostructures rather than isolated metalnanostructures. Since bulk metals as well as conformal metal surfacesmay be used, this bypasses the need for fine control of thin metal filmdeposition as well as vacuum-based metal deposition processes.

As disclosed herein, various embodiments relate to an all-metal,optically thick film having nanopits and protrusions of dimensions about100 nm to about 200 nm and a pitch of 250 nm, which are well below thediffraction limit, and which is adapted to provide high-resolutioncolors.

Proof-of-concept for generating a color palette using the processesdescribed above has been demonstrated. For a structure closelyresembling Design A shown in FIG. 3, Finite-difference time-domain(FDTD) simulations were performed using Lumerical (Lumerical FDTDsolution. http://www.lumerical.com/) with the optical properties ofaluminum based on data from Palik (Palik, E. D. Handbook of OpticalConstants of Solids, Vol. 1, 804 (Academic Press, 1985).

The simulation model was built off the schematic shown in FIG. 4(A).Arrays of 50 nm tall aluminum nanopillars formed on a 500 nm thickaluminum layer (which is equivalent optically to a bulk aluminumsubstrate) were simulated. The dimensions of the nanopillar spanned from140 nm to 200 nm, with each pillar spaced 250 nm away from the adjacentpillar. Geometry of the nanostructure studied here is similar to DesignA shown in FIG. 3(A), which comprises an optically opaque metal filmpatterned with either nanopits or nanoposts and coated with a polymerfilm. The simulated reflectance spectra plots for the nanostructures aregiven in FIG. 4(B). Clearly, a spectrum of different colors may beachieved through variation of the nanostructure diameter spaced aconstant distance apart (250 nm). These simulations hence indicate thefeasibility of engineering plasmonic color pixels using metalnanostructures formed on an optically opaque slab of metal. Subsequentsteps would be to fabricate these new types of plasmonic pixels andgenerate a color image by modifying a software code to create a layoutfor the electron-beam lithography system.

For Design B illustrated in FIG. 3(B), for the case when the mastermould approaches to the nanometric length-scale, it is possible to formnanoscale grooves using different geometric shapes fabricated on themetal surface. Here, gap plasmon modes may be excited within thenanoscale groove region, resulting in a dip in the reflectance spectrum.This dip corresponds to a specific color in the visible spectrum andwhich, may be viewed plainly under an optical microscope.

Preliminary experimental results showing the colors obtained fromnanoscale square, hexagonal and triangular grooves fabricated on agold-coated surface are presented in FIG. 5(B). Corresponding scanningelectron microscope images of the fabricated nanostructures shown inFIG. 5(c) to (E) indicate the typical groove size ranging from 10 nm to15 nm. When a broadband light is incident on the sample surface, a gapplasmon mode will be excited at its resonance wavelength with its energyconfined within the gap. Accordingly, this excitation will induce a dipin the reflectance spectrum as shown in FIG. 5(F). The dip in thereflectance spectrum, and therefore, the color exhibited by each pixelmay be tuned simply by changing the gap size. This is verified byFinite-difference time-domain (FDTD) simulations using Lumerical asshown in the reflectance spectrum for rectangular grooves with gapsvarying from 9 nm to 17 nm in FIG. 5(F). As such, each type of geometryused to form the nanogrooves may provide a new palette with differentcolors and tones.

In various embodiments, plasmon resonances from nanometallic structurescomposed of nanoposts or nanopits patterned with periodic structuressmaller than the wavelength of light are designed and exploited as astrategy for plasmonic-based high resolution color printing.

These plasmon resonances will manifest as a full range of colors visiblein bright-field microscopy. In addition, the colors, and thus imagesprinted, may be made to be sensitive to polarisation of the incidentlight. Furthermore, the colors visible in darkfield microscopy may bedifferent from that visible in bright-field. The above features provideadditional layers of encryption/encoding that will find use inapplications such as anticounterfeiting of currencies.

The colors exhibited in the structures as shown in FIG. 3A—metalnanostructures formed on an optically opaque metal film and coated witha polymer film may be controlled by the dimensions (e.g. width, heightand pitch) and geometry of the nanopost.

The colors exhibited in the structures for the case of FIG. 3B—bulkmetal substrate patterned with nanostructures may be controlled by thedimensions of the nanoposts (e.g. width, height and pitch), geometry ofthe nanopost (e.g. square, triangular or hexagonal) and the size of thegaps formed between the nanostructures.

Resolution of this method (minimum color pixel size) according tovarious embodiments is about 250 nm, which is well below the diffractionlimit. Methods disclosed herein will work with a range of other basicelements, including but not limited to the nanodisk or nanosquaregeometry. For example, linear structures and an array of holes withvarying density will also achieve a similar result.

This new architecture allows entire surface of the metal film to becontinuous, rather than individual metal nanostructures isolated fromeach other by another material such as a dielectric. It may also work ifa bulk piece of metal is patterned to have nanopits or grooves ornanoposts formed on its surface.

The architecture of this design fundamentally allows for large-scalecost effective production. While this method using e-beam lithographyhas been demonstrated, it may also work with high-resolutionphotolithography techniques. The method also allows for images to bereproduced in mass volume using nanoimprinting.

Methods disclosed herein may be used to create high-resolution colorprints which could find use as security features on items such as smartcards/credit cards, due to the high-level of technology involved increating the micro-image. Furthermore, unlike conventional pigment-basedcolor printing, methods disclosed herein have the capacity to engineercolor pixels that can encode of multiple layers of optical informationwithin the same area of volume. The approach for coloring metal surfacesproposed herein should also provide colors that have a wider viewingangle than diffractive methods. Such printing capabilities could findapplication in adding color to coins, jewellery, decorative metalfinishing.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. A process for plasmonic-based high resolutioncolor printing, the process comprising: a) providing a nanostructuredsubstrate surface having a reverse structure geometry comprised ofnanopits and nanoposts on a support; b) forming a conformal continuousmetal coating over the nanostructured substrate surface entirely togenerate a continuous metal film comprising a solid metal substrate andnanostructures on the solid metal substrate for the plasmonic-based highresolution color printing, wherein a periodicity of the nanostructuresis equal to or less than a diffraction limit of visible light; c)separating the continuous metal film from the nanostructured substratesurface by peeling the continuous film formed over the nanostructuredsubstrate surface entirely, from the nanostructured substrate surface;and d) providing a broadband visible light source so that plasmonicresonances generated by the nanostructures when the nanostructures areirradiated with the visible light from the visible broadband lightsource absorb one or more colors of light, and the nanostructuresreflect one or more other colors of light, thereby generating a colorimage, wherein the visible light is a broadband light.
 2. The processaccording to claim 1, wherein the support comprises a material selectedfrom the group consisting of a polymer, a metal, a metalloid, andcombinations thereof.
 3. The process according to claim 1, wherein thesupport is at least substantially transparent to the visible light. 4.The process according to claim 1, wherein providing the nanostructuredsubstrate surface comprises patterning the support by a method selectedfrom the group consisting of nanoimprint lithography, electron beamlithography, photolithography, and combinations thereof.
 5. The processaccording to claim 1, wherein providing the nanostructured substratesurface comprises patterning the support by nanoimprint lithography. 6.The process according to claim 1, wherein forming the conformalcontinuous metal coating is carried out by a method selected from thegroup consisting of vapor deposition, sputtering, casting, coating,electroplating, and combinations thereof.
 7. The process according toclaim 1, wherein the periodicity is 500 nm or less.
 8. The processaccording to claim 1, wherein the periodicity is in the range of about 5nm to about 500 nm.
 9. The process according to claim 1, wherein heightof each nanostructure is 200 nm or less.
 10. The process according toclaim 1, wherein height of each nanostructure is in the range of about100 nm to about 200 nm.
 11. The process according to claim 1, whereineach nanostructure has a cross sectional dimension in the range of about10 nm to about 250 nm, the cross sectional dimension being defined alonga transverse axis of the nanostructure.
 12. The process according toclaim 1, wherein the continuous metal film comprises a metal selectedfrom the group consisting of gold, silver, copper, aluminum, andcombinations thereof.
 13. The process according to claim 1, wherein thecontinuous metal film comprises aluminum.
 14. The process according toclaim 1, wherein the color image is observable using a bright-fieldillumination.
 15. A method for generating a color image, the methodcomprising irradiating visible light from a broadband visible lightsource on a nanostructured metal film or metal-film coated supportobtained by a process for plasmonic-based high resolution colorprinting, the process comprising: a) providing a nanostructuredsubstrate surface having a reverse structure geometry comprised ofnanopits and nanoposts on a support; b) forming a conformal continuousmetal coating over the nanostructured substrate surface entirely togenerate a continuous metal film comprising a solid metal substrate andnanostructures on the solid metal substrates for the plasmonic-basedhigh resolution color printing, wherein a periodicity of thenanostructures is equal to or less than a diffraction limit of visiblelight; and c) separating the continuous metal film from thenanostructured substrate surface by peeling the continuous film formedover the nanostructured substrate surface entirely, from thenanostructured substrate surface, wherein plasmonic resonances generatedby the nanostructures when the nanostructures are irradiated with thevisible light from the broadband visible light source absorb one or morecolors of light, and the nanostructures reflect one or more other colorsof light, thereby generating the color image; and wherein the visiblelight is a broadband light.